1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to optical modulators for generation of amplitude- and/or phase-modulated signals.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
The ever-increasing demand for transmission capacity and a variety of limitations on spectral bandwidth in optical communication systems bring about the use of “spectrally efficient” modulation formats. Such modulation formats are generally based on higher-order optical modulation, e.g., 8-ary and higher QAM (quadrature amplitude modulation) and APSK (amplitude- and phase-shift keying). Various subspecies of QAM and APSK are described, e.g., in (1) A. P. T. Lau and J. M. Kahn, “Signal Design and Detection in Presence of Nonlinear Phase Noise,” Journal of Lightwave Technology, 2007, v. 25, pp. 3008-3016; (2) M. Seimetz, et al. “Optical Systems with High-Order DPSK and Star QAM Modulation Based on Interferometric Direct Detection,” Journal of Lightwave Technology, 2007, v. 25, pp. 1515-1530; (3) J. Hongo, et al. “1-Gsymbol/s 64-QAM Coherent Optical Transmission over 150 km,” IEEE Photonics Technology Letters, 2007, v. 19, pp. 638-640; (4) K. Sekine, et al. “Modulation Parameter Tolerance for 8- and 16-APSK Signals,” Proceedings of Optical Fiber Communications Conference (OFC'06), paper JThB13, 2006; and (5) M. Ohm and J. Speidel, “Receiver Sensitivity, Chromatic Dispersion Tolerance and Optimal Receiver Bandwidth for 40 Gbit/s 8-Level Optical ASK-DQPSK and Optical 8-DPSK,” ITG-Fachtagung Photonische Netze, Leipzig, Germany, May 2005, pp. 211-217, all of which are incorporated herein by reference in their entirety.
A representative prior-art optical modulator suitable for higher-order modulation at a relatively high modulation speed (e.g., greater than 10 Gbaud) employs a relatively large number of light-modulating elements, each having a corresponding drive circuit. As a result, the overall complexity of the optics and electronics involved in the implementation of prior-art higher-order optical modulators is relatively high. Disadvantageously, this complexity renders those optical modulators impractical and/or cost ineffective.